In magnetic recording data storage systems, recording heads that use read sensors based on the magnetoresistive effect, called MR heads, have demonstrated capabilities of reading data recorded at very large areal densities on the magnetic recording medium. MR heads are becoming the technology of choice in both high and low-end magnetic recording systems, such as magnetic recording rigid disk drives. In spite of their present success their extendibility to yet higher areal recording densities is limited. An especially important limitation results directly from the basic physical effect at the heart of the magnetic field detection, i.e., the anisotropic magnetoresistance (AMR) of the material used in the read sensor. The magnetic field from the magnetic recording medium is sensed from a change in the resistance of the read sensor. In conventional MR heads the material used to form the read sensor is permalloy, Ni.sub.1-x Fe.sub.x (x approximately 0.19), for which the AMR gives rise to a small percentage change in resistance. A commonly used figure of merit, which estimates the signal capability of a given MR read sensor material, is the magnetoresistance coefficient, .DELTA.R/R, computed by dividing the change in resistance (for current parallel and perpendicular to the sensor magnetization) by the average resistance. A high .DELTA.R/R is thus desirable in magnetic recording systems that use MR heads. The magnetoresistance coefficient for bulk permalloy is only about 4.0%.
In magnetic recording disk drives, the thickness of the permalloy sensor layer in the MR head is constrained by the overall magnetic design of the head and disk, and essentially tracks the magnetic thickness of the disk. As the linear bit density in the disk increases, the magnetic thickness of the disk must be scaled down to reduce the transition width between adjacent magnetic bits, and thus with it the thickness of the MR sensor layer. However, the physical mechanism underlying AMR means that as the sensor layer thickness is reduced and becomes comparable to the mean free path of the conduction electrons, the magnetoresistance coefficient is also reduced significantly. This gives rise to severe limitations on the use of MR heads. For example, the MR permalloy sensor layer thickness required for a disk with 1 Gbit/in.sup.2 areal recording density is of the order of 150 .ANG.. This corresponds to a magnetoresistance coefficient of only about 2.0%, i.e., about half of the value for bulk permalloy. An even faster decrease in the AMR effect is observed as the MR sensor layer thickness drops below 100 .ANG., which is the thickness range required for disk areal densities exceeding approximately 5 Gbit/in.sup.2. FIG. 1 is a graph showing the prior art relationship between .DELTA.R/R and film thickness for permalloy films formed by conventional sputter deposition on substrates of 40 .ANG. tantalum (Ta) on silicon (Si), where the substrate temperature was approximately 50.degree. C. The data in FIG. 1 is for films using a substrate structure and room temperature deposition that is known to produce the state of the art highest achievable .DELTA.R/R. This relationship between .DELTA.R/R and permalloy film thickness can be approximated by a mathematical curve fit of data as follows: EQU .DELTA.R/R=A/(1+B/t) (1)
where A and B are constants and t is the permalloy film thickness.
An additional requirement of MR heads is that the MR sensor layer material exhibit low, preferably zero, magnetostriction. Magnetostriction (in actuality "saturation magnetostriction") is the fractional change in length, .DELTA.l/l, of the MR sensor layer material when the sample is magnetized to saturation from its unmagnetized state, where "l" is the length of the material sample in the direction of the applied magnetic field and ".DELTA.l" is the change in length of the sample. The magnetostriction must be close to zero in the MR head sensor layer because of uncontrollable stresses induced in the head during fabrication and lapping of the wafer on which the head is formed. These stresses result in strain in the material and consequently, through an inverse magnetostriction effect, alter the magnetic properties of the material. In particular, magnetic anisotropies can thereby be induced in the material. If the magnetostriction is negative this means that the magnetization preferentially aligns itself along the length of the MR sensor (because of the nature of the induced stress). A slightly negative magnetostriction is preferred. The problem is that the nature and magnitude of the stresses induced in the MR sensor layer are not predictable and therefore result in unpredictable magnetic properties. Since the MR sensor layer is magnetically soft, so that it is sensitive to small magnetic fields, any induced magnetic anisotropies can seriously degrade the performance of the MR head.
The MR sensor layer must also have a low value of anisotropy field (generally less than approximately 10 Oersteds). The anisotropy field is the field necessary to saturate the layer in the hard magnetization direction. A low anisotropy field is necessary for the sensor magnetization to rotate in response to the weak magnetic fields generated by the magnetic transitions recorded on the disk.
Permalloy films have been formed in the past by either thermal annealing the film after deposition or by heating the substrate during film deposition. These techniques and the results are described in IBM Technical Disclosure Bulletin, Vol. 15, No. 11 (April 1973), p. 3320; Journal of Electronic Materials, Vol. 2, No. 2 (1973), pp. 227-238 and IEEE Transactions on Magnetics, Vol. MAG-21, No. 5, September 1985, pp. 1563-1565. However, these techniques have not resulted in an increased .DELTA.R/R over the highest value achievable by deposition at room temperature and have also been accompanied by an unacceptable increase in magnetostriction and coercivities. It is generally understood that low easy axis and hard axis coercivities, and a low anisotropy field less than approximately 10 Oersteds (Oe) are desirable in MR permalloy sensor layers. A significant hard axis coercivity indicates a dispersion in the anisotropy field which may lead to noisy sensor response.
Thus what is needed is an MR head with a permalloy sensor layer that has an enhanced magnetoresistance coefficient to offset the signal loss that results from the necessary reductions in the thickness of the layer, so that an MR head can be fabricated that will function with extremely high areal densities on the disk. The MR sensor layer must exhibit this enhanced magnetoresistance coefficient at very low thicknesses to have the largest impact on current and future magnetic recording technology. The MR sensor layer must also have low coercivities and anisotropy field and essentially zero or slightly negative magnetostriction.